This invention relates to an arrangement for preventing the generation of unwanted spurious radio-frequency outputs by a television modulator such as those used in the home for modulating the output of a video game, computer, television camera or the like onto a standard television channel.
The common television receiver is adapted to select from among a number of television channels, and to receive within the passband of the selected channel a radio-frequency carrier signal modulated with video information which information may be displayed on a picture tube or kinescope. The kinescope and its associated drive circuitry are relatively expensive.
Of late, the marketplace has made available a number of devices which produce information in the form of video signals representing images or graphic information. Such devices include home computers, surveillance cameras and video games. These devices are normally not supplied with a self-contained visual display or with a remote video display device responsive to the video signals. Instead, the cameras, games and computers incorporate television carrier generators and modulators, by which a television-channel picture carrier is generated and modulated with the video signal containing the information to be displayed. Such a video source, carrier generator and modulator is described in U.S. Pat. No. 3,832,486 issued Aug. 27, 1974 to Wanek.
The Federal Communications Commission (FCC) has jurisdiction over devices which generate radio-frequency signals, and the manufacturers of video games, cameras and the like incorporating carrier generators and modulators must obtain approval of the devices from the FCC before marketing can begin. Obtaining approval is time-consuming and inconvenient, and may be injurious to the competitive position of the manufacturer of such a device if not quickly obtained.
Just as the manufacturers of video information devices find it economic to take advantage of the availability of a television receiver to the prospective user of their device, it would be desirable for the consumer who wishes to display video information to have but a single carrier generator and modulator. Thus, the consumer would be spared the expense of purchasing a carrier generator and modulator as an integral portion of each of the video information sources which he wishes to display. The manufacturers of video sources likewise would be spared the necessity of seeking approval from the FCC for each device. Such a stand-alone carrier generator and modulator (hereinafter referred to as a TV modulator) would have a video input terminal to which any of the numerous sources of video signals could be coupled by the consumer, and the video information contained in the signals would be modulated onto an appropriate television channel for selection and display by the television receiver.
With many sources of video information available, any one of which may be coupled to the video input terminal of the stand-alone TV modulator, the possibility arises that video signals having unacceptable characteristics such as excessive amplitude could be coupled to the video input terminal of the modulator. This might give rise to improper operation of the TV modulator. Excessive amplitude of the video input signal, for example, may result in the generation of unwanted spurious outputs outside the passband of the television channel in which it is desired that the information appear. Such spurious outputs may be coupled as is known in the art to nearby television receivers and interfere with reception on those receivers.
It is known to provide automatic control of the amplitude of the carrier signal generated by the carrier generating and modulating arrangement as described in U.S. Pat. No. 3,641,451 issued Feb. 8, 1972 to Hollingsworth, et al., and U.S. Pat. No. 3,866,136 issued Feb. 11, 1975 to Augustin, et al., but this cannot prevent the appearance of spurious signals outside of the prescribed passband in the presence of an excessively large modulating signal. It is also known to use an automatic gain control (AGC) for automatic control of the amplitude of a video signal applied to the modulator in order to prevent spurious outputs from becoming large, but such automatic gain control arrangements have only a limited range. If the limited range of attenuation of such an AGC system is exceeded by an abnormally large video signal applied to the video input terminal of the TV modulator, the signal applied to the modulator portion may be sufficiently large to allow the undesired spurious signals to be generated.
It is desirable to have a stand-alone TV modulator adapted to receive a video input from a wide variety of sources but which will not produce undesirable spurious signals regardless of the amplitude of the video signal applied to the modulation input.